I get my prime briskets from Sam's. I cooked one for Easter this year and before I did, I separated the point from the flat and trimmed almost all the fat off from both. I cooked both at 225-250 for about 24 hours, until the IT reached 203, and the result was very dry meat, both point and flat.
For labor day, I bought another prime packer from Sam's. I probably trimmed 1-2 pounds of fat off, but I left around 1/4 in. fat on the fat side, did not separate the point from the flat, and only trimmed a small amount where the flat meets the point. I did score the fat down to the meat every half inch or so. I cooked fat side up for about 20 hours at 225, IT 203 in point, and the results were excellent; the flat was just a bit dry but quite edible.
I won't be aggressive in trimming fat in the future.